1. Field of the Disclosure
Embodiments of the present disclosure relate to an organic light emitting display device, and more particularly, to an organic light emitting display device which facilitates to improve an efficiency in compensating degradation of a driving TFT, and a method of driving the same.
2. Discussion of the Related Art
FIG. 1 is a circuit diagram illustrating a pixel of an organic light emitting display device according to the related art.
Referring to FIG. 1, each pixel of a display panel may include a first switching TFT (ST1), a second switching TFT (ST2), a driving TFT (DT), a capacitor (Cst), and an organic light emitting diode (OLED).
The first switching TFT (ST1) is switched by a scan signal (or gate signal) supplied to a gate line GL. According as the first switching TFT (ST1) is turned-on, a data voltage (Vdata) supplied to a data line (DL) is supplied to the driving TFT (DT).
The driving TFT (DT) is switched by the data voltage (Vdata) supplied from the first switching TFT (ST1). A data current (Ioled) flowing to the organic light emitting diode (OLED) is controlled by switching the driving TFT (DT).
The capacitor (Cst) is connected between gate and source terminals of the driving TFT (DT), wherein the capacitor (Cst) stores a voltage corresponding to the data voltage (Vdata) supplied to the gate terminal of the driving TFT (DT), and turns-on the driving TFT (DT) by the use of stored voltage.
The organic light emitting diode (OLED) is electrically connected between a cathode power source (VSS) and the source terminal of the driving TFT (DT), wherein the organic light emitting diode (OLED) emits light in response to the data current (Ioled) supplied from the driving TFT (DT).
The organic light emitting display device according to the related art controls an intensity of the data current (Ioled) flowing from the first driving power (VDD) to the organic light emitting diode (OLED) by switching the driving TFT (DT) according to the data voltage (Vdata), whereby the organic light emitting diode (OLED) emits light, thereby displaying an image.
However, in case of the organic light emitting display device according to the related art, the characteristics of driving TFT (DT), for example, threshold voltage (Vth) and mobility may be differently shown by each pixel due to un-uniformity in a process of manufacturing the TFT. Accordingly, even though the data voltage (Vdata) is identically applied to the driving TFT (DT) for each pixel, it is difficult to realize uniform picture quality due to a deviation of the current flowing in the organic light emitting diode (OLED).
If video data (data voltage) is applied to the driving TFT (DT) for a long time, the threshold voltage (Vth) of the driving TFT (DT) is shifted due to stress. In order to compensate for the shift of the threshold voltage (Vth) of the driving TFT (DT), there are an internal compensation method and an external compensation method. In case of the internal compensation method, a compensation process is performed inside the pixel. Meanwhile, in case of the external compensation method, a compensation process is performed outside the pixel.
For the external compensation, a sensing signal line (SL) is formed in the same direction as a gate line (GL). The second switching TFT (ST2) is switched by a sensing signal (sense) applied to the sensing signal line (SL). The data current (Ioled), which is supplied to the organic light emitting diode (OLED) by the switching of the second switching TFT (ST2), is sensed by an ADC (analog-to-digital converter) of a drive IC.
In case of the external compensation, the threshold voltage (Vth)/mobility of the driving TFT (DT) may be sensed after blocking the current flowing in the organic light emitting diode (OLED). Then, sensing data may be generated by the sensing driving, and variations in the characteristics of driving TFT (DT) may be compensated based on the sensing data.
However, in case of the related art sensing compensation method, the driving TFT (DT) is sensed under the condition that there is no current flowing in the organic light emitting diode (OLED) by blocking the first driving power (VDD). Thus, when an image is displayed, it is difficult to apply the related art sensing compensation method.
In order to overcome this problem, the sensing signal is supplied to one horizontal line among all horizontal lines during a blank period (if it is driven by 120 Hz, about 360 us) between an (n)th frame and an (n+1)th frame, thereby performing a real-time sensing process.
During the blank periods of the plurality of frames, the pixels are sequentially sensed by each one horizontal line from the first horizontal line to the last horizontal line, thereby sensing the threshold voltage (Vth)/mobility of the driving TFT (DT) for all the pixels. After that, compensation data is generated based on the sensed threshold voltage (Vth)/mobility, and then data voltage (Vdata) applied to the pixel is compensated based on the generated compensation data.
However, in case of the related art real-time sensing method, it is difficult to obtain precise sensing data since it is very sensitive to the surroundings such as light or temperature. Also, during the sensing process, the current is not flowing in the pixel, whereby a luminance of the line performed with the sensing process is relatively decreased by 5% in comparison to that of the lines normally supplied with the current. Thus, the sensing line on a screen is discerned due to the relatively-low luminance by the sensing driving.
In order to overcome this problem, stress data of the driving TFT (DT) is accumulated by counting the video data, to thereby estimate a degradation level of the driving TFT (DT). Then, the compensation data is generated based on the estimated degradation level, and the external compensation is performed. That is, the compensation data may be generated by accumulating the stress data of the driving TFT (DT) without sensing the pixel.
FIG. 2 illustrates problems of degradation compensating method using the data counting method according to the related art.
Referring to FIG. 2, the degradation compensating method using the data counting method has the following problems. If a degradation modeling of the driving TFT (DT) is not precise, there may be errors in the compensation data. Even though the degradation modeling is precise, a counting value of the video is distorted if an image is displayed for a long time, and the distorted counting value of the video data is gradually increased in accordance with the elapse of time. Thus, if the error of the counting value of the video data is not compensated, errors occur in the compensation data.